Often you need to inject into the flight plan an along track waypoint, that is defined by a distance from a known waypoint or navigational aid. There are several ways to achieve this and each method is similar in operation, but is used in differing circumstances. Depending upon the software version installed in the CDU, you can either use the LEGS or FIX page.
To create an along track waypoint in the CDU
- In the LEGS page, type into the scratchpad the waypoint name, vector and distance. The distance is where the new waypoint will be created and is measured from the waypoint or navigation aid you are using as an anchor (LAV340/10).
- After entering the information into the scratchpad in LEGS (LAV340/10), upload to the CDU by key pressing the appropriate waypoint, where you wish the along track waypoint to be created.
- Clean up any discontinunity and execute.
- LAV is the waypoint, 340 is the vector, in degrees, that the along track waypoint will be created, And 10 is the distance from LAV that the new waypoint will be created.
- To insert the along track waypoint before the anchor waypoint, use the negative key - (LAV340/-10).
- To insert the along track waypoint after the anchor waypoint, do not use any identifier (LAV340/10).
- Take note that the / is after the vector, and the waypoint name and vector are joined with no spaces.
If you want the new waypoint to be created along the current navigation track, type the vector that is displayed adjacent to the waypoint in the LEGS page of the CDU. If you do not type a vector, then the location of the along track waypoint will be allocated automatically, which may not be along your current track course.
In the example shown (LEFT), I have created an along track waypoint called LAV02 which is located the aircraft side of the waypoint LAV. I also have created a circle around LAV (see instructions below).
The CDU software will create along track waypoints with generic names (LAV01, LAV02, LAV03, etc). In the example, I have already flown over LAV01 and LAV02 is now the next waypoint.
To create a circle around a waypoint using the FIX functionality in the CDU
There are many reasons why you may want to place a circle identifier around a waypoint or navigational fix at a specific distance. Least of which to highlight a distance from a waypoint so it can easily be seen on the Navigation display (ND).
- Press FIX on the CDU to open the FIX page.
- Type into the CDU scratchpad, the name of the waypoint or navigation aid (VOR, NDB, etc) and upload to the FIX page (in the example, LAV).
- Do one of two things:
- Type into the scratchpad the distance you require the circle to be drawn around the waypoint. For example, LAV/15 and upload this to LSL1, or
- Type only the identifier (LAV). This will only create a small circle around the waypoint.
A quick way to insert a waypoint from a flight plan is to press the waypoint name in the LEGS page. This will automatically down select the waypoint to the scratchpad saving you the time typing the name. Then select FIX and upload the waypoint from the scratchpad.
In the example shown (ABOVE), I have created a circle at a distance of 15 miles from LAV which can be viewed on the navigation Display (ND). The straight, green dashed line displayed from LAV is the vector, which in this example is unimportant, as we are only creating a circle.
The vector only becomes important when you want to create a waypoint that intersects the edge of the circle (see below).
How to create a single along track waypoint along the edge of the circle.
If you want to create an along track waypoint at the edge of the defined circle, and insert this waypoint in line with the aircraft's current navigation track, then the vector becomes more important. The vector will determine the bearing, from the waypoint anchor that the waypoint is created.
If you want to ensure that the inserted waypoint is in line with aircraft's current navigation track, use the vector that is shown in the FIX page. This will ensure that the new waypoint will be created along your current flight track at the distance entered (circle)
In the example (BELOW), I have created the first, along track waypoint (ADM 01) as an entry point to an arc, directly in line with the navigation track of the aircraft. The waypoint is at the edge of the defined circle.
To insert along track waypoints around the arc of the circle.
Another variable of the above theme, often used when executing an NDB Approach, is to create several along track waypoints that intersect a defined circle. The waypoints which are created at a set distance from an anchor waypoint or navigation fix, create an arc.
First, ensure you have a circle created around the waypoint at the distance required (FIX page).
- Select the waypoint for the arc from the LEGS page and download to scratchpad.
- Type in the scratchpad: ADMAR100/15, ADMAR130/15, ADMAR160/15, ADMAR190/15 and so forth. This will create an arc 15 miles from ADMAR.
- If you want the first waypoint to be along your navigation track, use the vector for this initial waypoint indicated in the LEGS page of the CDU.
In the example (LEFT), I have created a number of along track waypoints, separated each by 30 degrees, that intersect the circle 10 miles from ADMAR. With a little imagination, it's easy to uncover several uses for such funtionality: terrain avoidance, approach protocols, etc. Click image to enlarge.
It’s important to note that user and along track waypoints are given generic names by the software (ADMAR01, ADMAR02, ADMAR03, etc).
To ensure that the waypoints are sequential when displayed (01,02,03,04,05, etc), upload the new waypoints to the LEGS page, to the same waypoint name that was used to create the along track waypoint. In this example it is ADMAR.
Understanding the CDU
What I've described above is but a very brief and basic overview of some functions that are easily performed by the CDU.
CDU operation can appear to be a complicated and convoluted procedure to the uninitiated. However, with a little trail and error you will soon discover a multitude of uses. It's important to remember, that there are often several ways to achieve the same outcome, and available procedures depend on which CDU software you are using.
I am not a professional writer, and documenting CDU procedures that is easily understood is challenging. If this information interests you, I strongly recommend you purchase the FMC Guide written by Bill Bulfer. Failing this, navigate to the training section of this website and download some of the FMC tutorials.